assume cs:code
stack segment
    db 128 dup(0)
stack ends 
code segment
    start:
        mov ax,stack
        mov ss,ax
        mov sp,128

        mov ax, 0b800h
        mov es, ax          
        mov ah, 'a'
    s:
        mov es:[160*12+40*2],ah
        call delpy
        inc ah
        cmp ah,'z'
        jna s

        mov ax,4c00h
        int 21h
    delpy:
        push ax
        push dx
        mov dx,3h ;循环30000h次
        mov ax,0
    s1: 
        sub ax,1 ; (ax)=(ax)-1
        sbb dx,0

        cmp ax,0
        jne s1
        cmp dx,0
        jne s1

        pop dx
        pop ax
        
        ret

code ends
end start    

